A sliding device has been frequently used under high load and durability-requiring circumstances, such as a condition where a crankshaft of an automotive reciprocating engine is operated. Since it is possible to improve the work efficiency of the whole sliding device by reducing a sliding resistance, a variety of efforts has been hitherto made for the purpose of improving the work efficiency. In the sliding device where sliding is made in the presence of a lubricating oil, a measure for reducing the sliding resistance is to minimize a surface roughness, which has been presently continuously carried out. Recently, it has been possible to machine a surface so as to obtain a very small surface roughness under development of machining techniques.
For example, for the purpose of reducing a frictional force and improving a wear resistance, it has been proposed to minimize an average roughness along with change in surface film and material, as disclosed in Japanese Patent Provisional Publication No. 2004-28276 and 2004-138128. In the Publication No. 2004-138128, the surface roughness Ra is proposed to be not larger than 0.03 micrometer.
In a sliding device where sliding is made in high load circumstances, the material and hardness of one sliding surface are different from those of an opposite sliding surface from the viewpoint of obtaining a seizure resistance, in which it is found that the surface roughness of the sliding surface having a smaller hardness than the opposite sliding surface is larger than that of the opposite surface.
For example, in a crankshaft of an automotive reciprocating engine, a shaft is made of a carbon steel and machined to have a surface roughness Ra of 0.1 to 0.03 micrometer, while a bushing has a bushing metal made from metal such as bismuth, tin, silver, lead, nickel and/or like, alloy made from aluminum, tin, lead, copper and/or nickel, or polymer, the bushing metal having a relatively large initial surface roughness Ra of about 0.3 to 0.4 micrometer. In another hand, the choice of Ra has been used rather than Rq because of its larger use in the industry. In this case, adaptation of the bushing metal progresses during operation of the bushing metal, so that flattening of the bushing metal is made to a certain extent. In many cases, an effort to minimize the surface roughness of the shaft is made to reduce a frictional resistance; however, a consideration has hardly been made on the surface roughness of the side of the bushing. In spite of such a situation, a certain sliding resistance lowering effect has been able to be obtained. This is assumed to result from the fact that adaptation or running-in (meaning that the surface roughness of a contacting section becomes small owing to an initial sliding) of the above-mentioned bushing metal having a smaller hardness progresses even if the roughness of the bushing metal is larger.